1. Field of the Invention
This invention relates to incremental photo-electric measuring apparatus using the moire fringe concept of measurement. This principle is used in such devices as optical comparators, height gages and measuring machines. It also finds application in machine tools.
2. Description of the Prior Art
The main element of a moire fringe type measurement system is an accurately ruled grating or scale of the desired length of travel. This scale is commonly of glass or steel and has a precisely known number of lines per unit length (for example, 500 equal lines and spaces per inch). When a second scale with the same line structure is superimposed on the scale at a slight angle such that the two line structures cross a pattern of dark and light bands is produced as the lines on the second scale fall on lines or spaces of the first scale. This integrated image is what is known as a moire fringe pattern. The spacing of the moire fringes can be adjusted by rotating one scale relative to the other about an axis which is perpendicular to both scales. When the second scale, commonly known as the index grating, is translated along the scale normal to the direction of the lines on the scale the fringe pattern travels at right angles to the direction of movement. By passing a light beam through both the scale and the index grating the movement of the fringe pattern may be converted into electrical signals by the use of photoelectric sensors. The output from such photo-electric sensors can be, for example, summed, shaped and amplified and then fed to an electronic digital display counter. The signals from the photo-electric sensors may also be used to give an indication of the direction of motion.
As a general proposition, systems incorporating the moire fringe concept of measurement are not subject to wear errors or the degree of backlash inherent in translating the motion of, for instance, a rack and pinion to a direct measurement. Thus, they can be used on machine tools whenever a linear or angular change in position must be accurately determined. Such use eliminates the need for highly accurate lead screws, racks and pinions or other similar devices.
Measuring apparatus based on the moire fringe concept comprises three basic parts: (1) an accurately ruled scale; (2) a reading or transducer head which is movable relative to the scale; and (3) an electronic display unit for translating the electronic signals received from the transducer head into a legible numeric display. In many systems the transducer head includes an index grating, a light source and photo-electric sensors.
In order for the transducer head to pick up an optical image off the scale the spacing and alignment between the scale and the index grating has to stay very constant. In the past, this has required a lengthy and critical mounting procedure whereby the scale had to be lined up parallel to the direction of, for example, machine table travel and then the transducer head had to be separately mounted at a specified distance from and rotational alignment to the scale. Typically, the parallelism between the longitudinal axes of the transducer head and the scale had to be maintained within a few thousandths of an inch along the entire length of travel. If the system had to be dismounted for servicing such lengthy alignment procedure had to be repeated upon reinstallation.
As an alternative to the above described separate parallel mounting of the scale and the transducer head both the scale and the transducer head may be mounted to the same spar. In this type of system the scale is fixed to the spar and the transducer head (containing the grating scale) mounted on a slide. The slide may take the form of a pair of machined surfaces on the spar. However, in order to insure that the spacing between the scale and the spar remains constant over the entire length of travel the slide must be carefully machined and the scale carefully mounted parallel to it.
In the past, two methods were used to mount the spar which supports the scale to the machine or fixture on which the scale was to be used. One method consisted of mounting the spar at each end only. However, for spar lengths of 4 feet or longer, vibrations could easily be set up which could jeopardize the correct functioning of the system. Therefore, on longer lengths cumbersome support brackets were required to keep the vibrations to a minimum and also to prevent excessive deflection. Another method of mounting the scale supporting spar consisted of providing mounting holes at certain increments over the length of the spar. These mountings holes increased the height of the spar which, on certain machines, made mounting very difficult.
This invention overcomes the above described alignment problems by sliding the transducer head directly on the scale. Constant spacing and alignment between the two gratings is thereby assured. This invention also overcomes prior art mounting problems by providing the spar with a longitudinally extending female dovetail which is received on a male dovetail fastened to the mounting surface.